Electric circuit



2 shegts-sh'eet 1 B. M. OLIVER ELECTRIC CIRCUIT Filed sept. 11, 1941 June 29, 1943.

A T7' ORNE V B. M. OLIVER 2,322,875

ELECTRIC GRCUIT Filed sept. 11, 1941 2 sheets-sheet 2- June 29, 1943.

Patented June 29,194'3 Bernard M. Oliver, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 11, 1941, Serial No. 410,353

This invention relates to electric circuits and lmore specifically to circuits containing amplifier `stages and to methods of and means for correcting for distortions therein.

It is an object of this invention to provide novel methods of and means for correcting for 21 claims. la. 179-171) y pensating for this distortion was confronted in distortions in an amplifying or repeating circuit.

It is another object of this invention to prol vide a novel sweep circuit containing an amplifier in which distortions introduced by the amplifier are compensated.

In the operation of a cathode ray tube suitablev for television, the sweep circuit used `is frequently of the type in which one or more condensers are charged and discharged in such a manner that a saw-toothed wave form of current or voltage isthe design of an amplifier to meet the abovementioned requirements for amplifying a sawtoothed current wave.

In accordance with the invention, a sweep .cirbuit is provided in which distortions in the output 'wave produced by large swings in amplitude in the output stage of the amplifier are largely compensated by balancing the effect of the curvature of the characteristic o f the 'iinal stage against that of the preceding or driving stage by i loading the driving stage so that the working produced, which wave form may be applied be- 4 'tween a pair of electrostatic defiecting plates` within the tube or through electromagnetic defleeting coils associated with the cathode ray tube. It has often been found in practice 'that it is desirable to amplify the saw-toothed waves before applying them to the deflectingmeans. The design of an amplifier suitable for passing a sawtoothed wave introduces many problems due to I sired amplifier must thus be capable of passing awide band of frequencies with minimum distor-l tion and at the same time to deliver to the output circuit (the deflection coils or al transformer feeding them) a large current. The quality of a television image i's kgreatly affected by even small departures from linearity in the saw-toothed deflecting waves so that substantial elimination of distortion in that portion of the amplifiedv defiectfng waves which effects the scanning movements of the cathode beam is highly desirable.

There is of course an advantage in conning the working range. of an amplifier to the straightest portion of its input-voltage outputcurrent characteristic, but when large output is desired it may be expedient to permit the workrange likewise extends beyond the straighter portion of its input-voltage output-current characteristic. In the arrangement chosen to illusvtrate the invention the necessary loading of the driving stage is accomplished by making the output or anode resistance small', utilizing the poteng tial difference across this resistance as the input to the final stage, and using an input amplitude for the driving stage suiliciently large` to give the required input voltage for thefinal stage.

There still exists after this compensation a slightV ing range to be extended in the final or power stage, so that distortion is introduced by the curvature of the characteristic beyond its relatively straight portion, if compensation for this residual distortion (S curvature) which, in the illustrative arrangement hereinafter described,

is substantially compensated by introducing into the input of a stage prior to the driver stage and in reverse phase 4to the signal component of fundamental'frequency, some of the fundamental frequency which, for a saw-tooth wave used for line scanning deflection in a 441-line television transmitter tube, is about 13 kilocycles. The method of obtaining sweep linearity in accordance with the present invention is particularly useful when the design of the transformer feeding the sweep coils or any other factor makes the use of a'push-pull output stage undesirable.

It will -be noted that the introduction of a wave of fundamental frequency and of small amplitude. relative to that of this component of the saw-.toothed wave, into the input of the earlier amplifier stage in phase opposition to that of the component of fundamental frequency of the signal, reduces the amplitude of the component of fundamental frequency of the signal. 'I'he same result may therefore be obtained by any means which reduces the amplitude of the fundamental frequency component of the saw-toothed signal wave by the proper amount. It is preferred to effect this reduction before the signal wave has been amplified to a large value, in which case the` reduction results` in a predistortion o f the signal wave.

- The invention will be more readily understood ldistortion can be effected. This problem of comby referring to the following description taken in ure there is provided a sweep circuit. I3 which may be considered, for convenience, as divided into a sweep generator A and an amplifier B.

The sweep generator A-comprise's a sweep condenser I I, two tubes Vi and Vn comprising a twotube synchronized multivibrator arrangement to steepen the wave front,V and a constant current device Va. The amplier B preferably comprises three stages, the'iirststage consisting of the tube V4, the second stage of two tubes Vs and Vn in parallel, and the third stage consisting of three tubes V1, Vs and V9 in parallel.

With reference to the sweep generator A, the sweep condenser .is charged relatively quickly by means of the multivibrator (the tubes V1 and V2) and discharged relativelyslowly and substantially linearly with respect to time by means of the constant current device Vs.

The tube V1 preferably comprises an :anode 23.` a cathode 2|, a. control grid 22, a screen grid 23, and a suppressor grid 24. Self-bias for the control grid 22 is supplied by means of the condenser 25 and the resistance 26 connected thereacross, a. grid resistor 21 being also connected in circuit between the cathode 2| and the control grid 22. synchronizing signals oi', for example, line scanning irequency,' are applied between the control grid 22 and the cathode 2| from an input circuit 28 by means of a coupling resistor 23 and a coupling condenser 33. The suppressor grid 24 is connected to the cathode in accordance with wellknown practice. The screen grid 23 is connected through resistances 3| and 32 to the positive terminal of a source 33, a condenser 34 being connected from the junction of the resistance 3| and the resistance 32' to ground to provide a low impedance path for alternating current. 'Ihe screen grid 23 is also connected through a coupling condenser `35 to the anode 4| of the tube Vn which preferably comprises Ithe anode 4|, a cathode 42,

and a control grid 43a The anode 23 of the tube tube is inherently a constant current device over a large portion of its characteristic, the sweep condenser discharges substantially linearly and slowly through the device V3, it being charged periodically and relatively quickly by the multivibrator arrangement V1 and V2, as will be pointed out more fully below.

The cathode 42 of-the tube V2 is connected by means of the wellknowncathode followerfconnection to the control grid 62 of the tube V4 through a coupling condenser 65, a coupling re- Vils connected through an anode resistor 33 and y the high potential source 33 to ground and is also connected directly to the grid 43 of the tube V2. The cathode 42 of the tube Va is connected through the sweep condenser I to ground and is connected directly to the anode 53 of tube Vs. Thev anode 4| oil the tube Vz is connected through an anode resistor 44 and the high potential source 33 to ground.

. The constant current device V3 preferably comprises an anode 33,'a cathode 5|, a control grid 52, a screen grid 33, anda suppressor grid 54. Bias for the cathode 5| is supplied by means of the resistor 55 and the condenser 5 6 which are connected between the cathode 5| and ground,

`sistor 66 being also included in the input circuit of this tube.. The tube V4 preferably comprises an anode 63, a cathode 6|, the control grid 62, a screen grid 63, and a suppressor grid 64. The

cathode 6| is connected through a resistance 31A and a tuned circuit C to ground. 'I'he tuned circuit C preferably comprises an inductance 63, a. condenser 63, and a variable resistor 13. This circuit C is tuned to be antiresonant at the fundamental frequency, which is about 13 kilocycles in a preferred operating arrangement. The screen grid 63 is connected through a resistor 1| and source 33 to ground while a low impedance path to ground for alternating current is'provided by means of the condenser 12. The suppressor grid 34 is preferably connected to the cathode 6|. The anode 63 is connected through an anode resistor 13 and the high potential source 33 to ground and is connected through a coupling condenser 14 to the second stage of the amplifier comprising the two tubes Vs and Vs connected in parallel. I

The tube Vs preferably comprises an anode Il, a cathode 3l and a control grid 82 while the tube Va preferablycomprises an anode 33. a cathode 34 and a control grid 35. Input resistors 33 and 31 are provided between the coupling condenser 14 andthe control grids 32 and 35', respectively.v

A commoninput resistor 33 is connected from thecommon terminal of the two resistors 36 and 31 to ground while self-bias for the cathodes 3| and 34 is obtained by means of the resistor 33 having the condenser 33 connected thereacross.

The anodes 3.3 and 33 are connected through output resistors 3|, 32 and a common resistor 33 to the positive terminal ofthe 300volt power supply 33, the negative terminal of which is connected to ground, these resistors being of excessively low resistance for a reason which will appear more'fullyhereinafter.

The output circuit of the tubes Vs and Vs is connected to the input circuit of the third amplifying stage comprising ,tubes V1, Va and Vs by means of coupling condenser 34. The tube V1 comprises an anode` 95, a cathode 3B, a control grid 31, a screen grid 33 and a beam forming plate 33. The tube Va comprises an anode |33,

l a cathode |3|, a control grid |32, a screen grid the control grid 52 being also connected to" ground. The suppressor grid 54 is directly connected to 'the cathode 5| while the screen grid cathode path of the tube Vais connected in circuit with the sweep condenser and as the pentode vconnected to the respective cathodes.

|33 and a beam forming lplate |34, while the tube V comprises an anode |35, a cathode |36,-

a control grid |31,l a screen grid |33 and a beam yforming plate |33. Grid resistors ||3, and ||2 are provided for the control grids 31, |32 and |31, respectively, these resistors having a common terminal which is connected to the coupling condenser 34. The cathodes 36, |3| and |33 are connected together and the common terminalv is connected'through the resistor H3 and the condenser ||4 connected thereacross to ground. The beam forming plates 93, |34 and |33 are 'I'he screen grids '33, |33 and |33 are connected through resistors H5, IIB and ||1, respectively, 30 common terminal ||3 which is connected to the source through the resistor |3I, and through a resistor ||8 to the positive terminal |20 of a high potential source which may be of the order of 2,000 volts. The resistor |3| and resistor ||9 are portions of a potentiometer resistor between ground and a point having a potential o f +2000 volts', this difference of potential being obtained by any suitable direct current power supply. 'I'his resistor H8 isalso connected to one terminal of the primary winding |2| of the transformer |22, the secondary winding |23' o f which is connected to sweep coils associated with a cathode ray tube. Preferably a resistor |24 shunts the winding |23. The anodes 85, and |05 are connected through resistors |25, |26 and |21, respectively, to a common terminal |28 which is connected through the primary winding |2| to the positive terminal |20 of the source of high potential.

Due to the large current swings in the last stage of the ampliiier, the output is not linear. However, this non-linearity can in large part be corrected by balancing the curvature of the second stage Vs and Vs against the curvature produced bythe third stage Vv, Vs and Vs. One way of doing this is by using unusually low load resistances 9|, 82 and 93 in the plate circuits of i the second stage so that to .obtain the same output. voltage it is necessary to drive the tubes of this stage over a larger "portion of their characteristic. ,While this predistortion or compensation largely corrects for the distortion which would otherwise be present in the output of the third stage, there still remains a slight S-shaped curvature in the over-all characteristic. The

-nature of the saw-toothed waveis such that when it is amplified by an amplifier having this S-shaped characteristic the resulting distortion which reduces the gain at 13 kilocycles (the frequency of the sweep circuit).

The operation-of the circuit shown in Fig. 1 is as`follows: Negative pulses from the synchronizing generator, not shown, and supplied over the input line 28, areflrst amplified by the tube V1 which may be a type B10-A pentode tube.-

Resulting positive pulses on the anode 20 of the tube are applied to the grid 43 of tube V2 which may be a type 262-B triode which thereby periodically and quickly charges the sweep condenser in its cathode lead. In order to shorten the charge time, a feedback voltage consisting of negative pulses is developed inthe plate circuit of the tube Vz and applied` to the screen grid 23 ofthe tube V1. yThis regenerative action steepens the wave front of the pulse on the grid of the tube V2 and thus shortensfthe charging time of the condenser in its cathode lead. The feedback is applied to the screen` grid rather than to the control grid in order to prevent pulses from the tubeVz from reaching the pulsing generator. The sweep condenser is also connected to the' plate of the tube Va which may be a 31o-A pentode tube, which tube is adjusted frequency o! the incoming pulsesover the cable The saw-tooth voltage developed across the` sweep condenser is f ed into the three-stage amplifier B which drives the sweep coils through the transformerml22. amplifier consists of the singletube V4 which may be a type 336'pentode tube. 'Ihe output oi' this stage drives the two tubes Vs and Ve in parallel which may b`e 271-A triode tubes and they in turn drive the three parallel output tubes Vv. Va'and Vn which may be'813 output tubes. The large amount of power in the output is necessitated by the high frequency` of the sweep (due to the fact that this sweep is particularly adapted for use with a 441-line dissector film scanner which would necessitate a sweep frequency o f over 13 kilocycles. -This power is also required because `a, considerable flux density must be established over a large space in the dissector tube. As pointed out above, in order to correct for the non-linearity of the output tubes V1, Vu

and Vs, certain predistorting characteristics are included in the first two stages of the amplier B. By using excessively low load resistances 8|,

82 and 93 in the plate circuits of the 27l'-A driver.

stage, so that it is also necessary toidrive this stage overa considerable portion ,of its characteristic to obtain the same output voltage, an opposite curvature in the input wave form introduced on the grids of the three output tubes Vv, Va. and Vs is obtained. This predistortion largely corrects for, the distortion which would otherwise be present in the output. However, there still remains a slight S-shaped distortion which, in the embodiment of the invention being described, is substantially removed by introducing some of the fundamental frequency in opposite phase to the component of fundamental frequency in the signal by 4means of the antiresonant local feedback impedance C in the cathode lead of tube V4, which reduces the gain at 13 kilocycles. This method of obtaining sweep linearity has provenl quite satisfactory and has produced a large-amplitude, substantially linear sawtoothed current wave in the coils of the dissector tube. The output wave may be fed through sweep coils of the type disclosed in Patent 2,278,478, issued April 7, 1942 to B. M. Oliver.

The manner in which distortions are removed in .the amplifier is graphically shown in Figs. 2 to 7, inclusive. Fig. 2 shows the characteristic curve of the third (last) stage alone; Fig. 3 shows the characteristic curve of the second (next-tothe-last) stage alone; and Fig. 4 shows the combined or over-all characteristic of the last two stages.4 It should be noted that thev curve in Fig.4 4 has a slight'S-curvature which is compensated by thel feedback in the first stage. Fig. 5

shows the manner in which the fundamental the-last stage to obtain an output wave which,

as shown in Fig. 7, is applied as an input to the last stage to obtain a desired output wave which is a linear saw-tooth.

.In a circuit constructed and operated in ac- The first stage of this l cordance with this invention the following values of circuit constants were used: Resistor 29, 75 ohms; resistor 21, 2,000 ohms; resistor25, 1,000 ohms; resistor 3|, 10,000 ohms; resistor 32, 0.2 megohm; resistor 35, 25,000 ohms; resistor M,

3,000 ohms; resistor 55, 50,000 ohms; resistor 51,

50,000 ohms; resistor 59, '0.1 megohm; resistor 66, 1 megohm; resistor 61, 20,000 ohms; resistor 10, 2,500 ohms; resistor 1|, 0.2 megohm'; resistor' y13', 10,000 ohms; resistors 86 and 81, 100 ohms 0.001 microfarad; condenser 2,5, 25 microfarads;

condenser 34, 1 microfarad; condenser 35, 0.001 microfarad; sweep condenser 0.0005 microfared; 'condenser 56, 1 microfarad; condenser-58, 1 microfarad; condenser 65, .0.001 microfarad; condenser 69, 0.03 microfarad? condenser 12, 1

` `microfarad; condenserv 14, 0.09 microfarad; condenser 90,25 microfarads; condenser 9L 0.5 microfarad; condenser ||4, 50 microfaradsi; and the inductance 68, 10 millihenries. Obviously,

however, the invention is not limited to a circuit having these values, they being given merely by way of example of an operative circuit.

Various modifications maybe made in the embodiment described herein without departing from the spirit of the invention, the scope of which is indicated in the appended claims. For

' example, the tubes in the driver stage may be of low current capacity,thus not requiring a low output resistanceto produce the required predistorting. Also, as above stated, the reduction in amplitude of the fundamental frequencyl component may be effected elsewhere than in the position chosen herein for purposes of illustration and by other well-known means for obtaining a reduction of onel frequency component relative to others or amplification of said other frequency components relative to said one.

What is claimed is:

1. A multistage amplifier comprising means -in an Aearly stage forproducing a distortion of a wave which is substantially complemental to that produced in a later stage in said amplifier, and means for correcting for the residual distortion resulting from the imperfect matching of said two frequency in the input circuit of one of the stages of said amplifier.

` 4. The combination of claim 3 in which said means in said stage preceding the last one comprises a relatively low output impedance.

5. The combination of elements asin claim 3 in which said means for introducing a wave of fundamental frequency comprises a tuned circuit in both the input and output circuits of one of the stages of said amplifier.

6. The method of' eliminating distortion of a signal wave in an amplifier in which, due to the wide range of amplitude values, distortion is introduced in the output stage, comprising the steps of predistorting the Wave in the next-to-the-last 'stage so that the naldistortion is at least substantlally compensated, and also applying another predistortion for compensating for imperfect compensation due to said first predistortion.

'1: The method of eliminating distortion of a' signal in an amplifier in which, due to the wide 8. The method of claim 7 in which said wave of fundamental frequency is eifectively introduced in a stage preceding the next-to-the-last stage.

9. I'he method of compensating for an sshaped distortion in the slowly varying portion of a saw-tooth wave which comprises the step of effectively combining a second wave of the fundai mental frequency of lsaid saw-tooth wave with distortions, said means comprising means for effectively introducing a wave of fundamental frequency in the input circuit of one of the stages of said amplifier.

2. A multistage amplifier comprising means in w the next-to-the-last stage for producing a diswaves comprising means ln a stage preceding the last one for producing a distortion of a wave l which is substantially complemental to that produced in the last stage of s'aid device, and means for correcting for the residual distortion resulting from the imperfect matching of said .two characteristics, said means comprising means for effectively introducing a wave of fundamental said saw-tooth wave. i

10. The method of claim 9 in which said second wave is a sine wave of relatively low amplitude in comparison with the peak amplitude of said saw-tooth wave. l

11. A circuit for amplifying saw-tooth waves suitablefor causing deflection of the beam in a cathode ray tube comprising amplifying means which introduces distortions into said wave, which distortions take the form of an S-shaped variation from linearity in theslowly varying portion of said wave, and means for effectively introducing 'a wave of the frequency of saidsawtooth Wave into said circuit to compensate for said S-shaped distortion. I

12. A circuit as in claim 11v in which said amplifying means comprises an output stage and a next-to-the-last stage both of which produce compensating distortions which are imperfectly matched.

13. The combination withI a ldevicefor repeatingslgnal waves having a voltage-input currentoutput characteristic comprising a substantially symmetrical S-shaped portion, of means for impressing an alternating non-sinusoidal signal voltage wave upon the input of said device so that the positive and negative portions of.said wave respectively fall wholly on the two sides of the center of symmetry of said `8 portion of the characteristic, and additional means for reducing the amplitude of the fundamental frequency comf ponent of said signal wave, before or after being repeated by said device, relatively to the other component or components of said wave to compensate in part at least for the distortion introduced by said device into the repeated wave.

15. A circuit for amplifying a signal wave com` prising an output stage and a driver stage, said output stage comprising a plurality of tubes in parallel and said driver stage comprising a plurality of tubes in parallel but having a lesser number of tubes than said output stage, means for 'applying an alternating current wave to said driver stage in such a manner that the range of amplitude of said wave covers 'a portion of the grid-volts vs. plate-current characteristic' of said driver stage which iscurved in one direction only, means for applying the output wave from said driver stage to said output stage in such a. manner that the range of amplitude of said lastmentioned wave covers a portion of the grid-volts vs. plate-current characteristic curve of said output stage which occupies a similar position on said curve as the operating portion of the characteristic curve of said driver stage corresponding to the range of amplitudes of the wave applied to said driver stage, whereby the distortion produced in said driver stage due to the curvature of its characteristic is in some measure compensated by the distortion produced in said out- Put 'stage due to the curvature of its characteristic but there remains, in the absence of other compensating means, a residual distortion having a frequency equal to the fundamental frefluency of said alternating current wave, and means for eectively introducing a compensating distortion of the fundamental frequency into said amplifier to produce a wave in the output of said output stage which isa substantially undistorted V counterpart oi' said signal wave.

16. The combination of elements as in claim 15 in which said last-mentioned compensating distortion is effectively introduced into said signal wave by means of a negative feedback 'circuit in a stage priorto said driver stage, said feedback circuit being antresonant at said fundamental frequency.

17. A circuit for amplifying saw-tooth waves having a slowly varying portion occupying most of the cycle suitable for causing deflection of the beam in a cathode ray tube comprising amplifying means which in the absence of compensating means introduces distortions into said wave, which distortions take the form of an S-sha-ped variation from linearity in the slowly varying portion of said wave, and means for compensating for said S-shaped distortions, said last-mentioned means-comprising means for reducing the amplitude of the fundamental frequency of said saw-tooth wave relative to that of other frequencies in said wave. I e

l 18. The combination with a wave transducinl means having an input-output characteristic each of two adjacent portions of which has a curvature substantially reverse to that of the other, of y means for impressing an alternating nonsinusoidal wave upon the input of said transducing means so that the positive and negative portions of said wave respectively fall substantially wholly upon said two portions of the characteris tic, and means comprising wave'selecting means selective of the fundamental frequency of said wave to change the amplitude of the fundamental frequency component of said wave relative to the other components thereof to compensate in part at least for the distortion introduced by the curvature of said characteristic.

19. The combination of claim 1s in which said transducing means comprises an even number of repeater stages in tandem.

20. The combination with two wave repeating elements in tandem each having an input-output characteristic having a portion which is curved, of means for impressing a non-sinusoidal wave upon the input of the first of said elements so as to cause it to repeat said wave while working upon said curved portion at least of its said characteristic, coupling means between said elements to impress the repeated'wave upon said second element so as to cause it to repeat said wave while working upon its said curved portion at least of its said characteristic, said two characteristics acting in a complemental manner to produce an over-al1 characteristic of generally S-shape and of relatively great curvatures, and means comprising wave selecting means selective of one at least of the frequency components of said nonsinusoidal wave to change'the amplitude of the component of said wave of which said selecting means is selective relative to other components thereof to compensate for distortion introduced by said over-all characteristic.

, 21. The combination with two wave repeating elements in tandem each having an input-,output characteristic having a` portion which is curved,

of means for impressing a saw-tooth wave uponl elements solas to.

the input of the first of said cause it to repeat said wave while working upon said curved portion at least of its said characteristic, coupling means between said elements to impress the repeated wave upon said second ele.

ment so as to cause it to repeat said wave while y working upon its said curved portion at least of its said characteristics, said two characteristics acting ina ccmplemental manner to produce an over-all characteristic having an s-shape of relatively great curvatures, and means comprising wave selecting means selective of the fundamental frequency of said wave to change the amplitude of the fundamental frequency component of said wave relative to the other components thereof to compensate for distortion introduced by the curvatures of saidA s-shaped characteristic.

` BERNARD M. OLIVER. 

